Casting paper or molding paper is used in the casting or molding of plastics to impart a textured surface. For example, PVC coated cloth can be embossed through the use of casting paper to form imitation leather. Casting paper can also be used for casting blocks of polyurethane as required principally in the furniture and automotive industries. Casting paper generally has a release surface, smooth or carrying a negative or reverse of a pattern (emboss) required in the final substrate (e.g., artificial leather). For example, when forming artificial leather, the casting paper can be used by extruding thermoplastic polyurethane or a polyvinylchloride plastisol onto the release surface; this is then dried or cured on the casting paper. The polyurethane or polyvinylchloride plastisol can then be transferred to a cloth surface to form the artificial leather. The artificial leather, carrying the positive impression of the original embossing roll, can then be stripped from the surface of the casting paper.
As such, casting paper needs to meet very severe requirements of heat resistance, clean stripping and repeated use, while retaining its embossed surface. One of the materials preferred in the art for use in forming the release surface is polymethylpentene (e.g., TPX from Mitsui Chemicals), which shows especially good heat resistance compared to other thermoplastic polymers. Polymethylpentene has been in use since the mid 1970's, but it is very expensive. Also, it can distort under high pressure or when heated at temperatures above about 350 degrees F. Highly crosslinked coatings are generally used if better heat resistance is needed.
In a typical process of forming casting paper, a release coating is coated onto the paper and texturized utilizing an embossed drum. The hard embossing roll has protrusions or knobs disposed in a desired pattern thereon to press into the surface of the coating. When the thermoplastic polymer polymethylpentene is the release coating, the coated paper is embossed against a heated drum and then simply cooled. The highly crosslinked release coatings are formed by first applying a curable liquid, which can contain a polymer or polymer precursor. The polymer or polymer precursor coating can contain water or solvent which is evaporated prior to curing or it can be 100% non-volatile. The paper with the curable coating is then pressed against an embossing drum and cured before the paper removed, giving a patterned coating which is heat resistant. However, these embossing drums are very expensive to produce. Therefore, the production of casting paper with a given pattern is not economical unless a particular drum is used to produce large volumes of casting paper with that particular pattern. Thus, changing the pattern formed in the release surface of the casting paper in this manner is expensive, effectively prohibiting the development of readily customized casting papers.
As such, a need exists for an affordable, more flexible method for forming casting papers, which will then make a wider variety of customized casting papers readily available.